The present invention relates to lubrication systems for turbine engines and for associated equipment, and more particularly, to air and lubricant heat exchangers for use in maintaining desired temperatures of the lubricants in such engines and equipment.
Lubrication systems for turbine engines, such as a turbofan engine, and for associated equipment, such as an integrated drive generator, provide pressurized lubricant, an oil, to lubricate, cool and clean the engine main bearings, gear box gears, and the like, and again for the lubrication of bearings and other parts in equipment associated with such turbine engines. During such lubrications, heating of the lubricant is caused to occur due to mechanical energy losses in the lubricated apparatus. Thermal management of such lubricants is very important for continued successful operation of such lubrication systems in the apparatus lubricated thereby.
The amount of heat necessary to be ejected from lubricants in such systems is increasing because of the use of larger electrical generators, for instance, in aircraft turbine engines due to increasing consumption of electrical power in the aircraft powered thereby, and because of the advances in aircraft turbine engines such as the use of geared turbofans for such aircraft with a large fan-drive gearbox. Despite the added heat generated by the such modified and expanded equipment, the necessary lubricating oil operating temperature ranges to provide satisfactory lubricating performance have not changed for the most part and, in some instances, the upper operating temperature limits have been reduced.
The lubrication system for a turbofan engine in an aircraft typically has a first heat exchanger providing lubricating oil passing through passageways in that heat exchanger that is cooled by the fuel stream flowing past these passageways. This arrangement permits the lubricating oil to reject heat therein to the fuel in the aircraft thereby heating that fuel to help prevent the occurrence of icing therein. Because in some flight situations more heat is generated in the lubricating oil than is needed for warming the fuel, a portion of the lubricating oil can be forced to bypass the heat exchanger for the lubricating oil and the fuel and be directed to a further heat exchanger, where the heat therein is transferred to the air in the secondary airstream provided by the fan of the turbofan engine.
In a typical arrangement, a duct is provided in the fan cowling through which a portion of the airstream is diverted, and the air and lubricating oil heat exchanger is placed in this duct so that the lubricating oil passing through passageways in that heat exchanger is cooled by the duct airstream flowing past these passageways in the exchanger. If such additional cooling of the oil is not needed in a flight situation, the lubricating oil can again be forced to bypass this air and lubricating heat exchanger.
However, the fan airstream diverted to pass through the lubricating oil and air heat exchanger in such duct systems always flows through that exchanger. Further, the duct cross sectional area and the heat exchanger passageways exposure to the duct airstream must always be sufficiently large to assure sufficient heat transfer to the airstream in the most difficult flight conditions encountered, and so are much greater in size than what is required in the great majority of flight conditions. Thus, such an air and lubricating oil heat exchanger duct based system continually leads to thrust losses in the turbofan engine despite being unnecessary for cooling the lubricating oil in many flight situations. Hence, there is a strong desire for an air and lubricating oil heat exchanger system that reduces such thrust losses and also reduces the volume required therefor in the more compact spaces in advanced turbofan engines.